Rendering of Seagen’s new planned manufacturing facility north of Seattle. (Seagen Image)Seattle-area biotech experts are eying the past with a wary eye in the wake of this week’s news that Pfizer plans to acquire Bothell, Wash.-based Seagen for a whopping $42 billion. Most previous acquisitions in the Seattle region have ultimately gutted biotech companies and their workforces, including at Immunex,…